1. Field of the Invention
The present invention relates to a method and apparatus for assessing cuttings from a wellbore. In particular, the present invention relates to a method and apparatus for collecting cuttings and for measuring the emissions from the cuttings so that the original depth of the cuttings within the wellbore can be determined.
2. The Prior Art
To determine the mineralogy of a well, cuttings from the wellbore are collected and analyzed. The cuttings are cut by a drill bit and are transported to the well surface by a drilling mud. The drilling mud is pumped into the well through the drill string and is returned in the annulus between the drill string and the wellbore. The cuttings are typically separated from the drilling mud by screens or sieves, gravity settling, centrifuge, or elutriation techniques.
Before the stratigraphy of a well can be assessed, the original depth of the cuttings within the wellbore must be determined. This correlation of a cutting sample with the original depth within the wellbore is difficult and is affected by numerous factors such as the volume of the wellbore and the mud pumping rate, annular velocity, and profile. In addition, different sized cuttings are transported by the drilling mud at different rates. Smaller cuttings move at a velocity close to that of the drilling mud, while larger particles are slowed by gravity and by other factors. The terminal velocity of a cutting particle within the drilling mud depends on the particle size, shape and density. The velocity is typically expressed as a cuttings transport ratio defined as the velocity of the cuttings divided by the velocity of the drilling mud. Differences in the transport ratio for different size cuttings cause the cuttings from a particular wellbore elevation to be dispersed across a range within the drilling mud. This dispersion further causes cuttings from one wellbore elevation to overlap with cuttings from a different wellbore elevation.
The size of the cuttings from a wellbore elevation depends on the formation hardness and other physical properties of the formation, on the style of drill bit, and on the rate of penetration. For example, polycrystalline diamond compact (PDC) bits shear and fracture the formation without regard to grain boundaries. Consequently, PDC bits create small cuttings which do not represent the original texture of the rock. The rate of penetration also affects the transport of cuttings. A high rate of penetration by the drill bit releases cuttings into the drilling fluid at a faster rate, and contributes to the overlap of cutting distributions from one wellbore elevation to another.
Other variables affect the calculation for the original wellbore elevation of a cutting sample. For example, the flow rate of the cuttings within the wellbore annulus, and the length of time necessary to clean the cuttings all affect the depth calculations. In addition, measurements of cutting depth can be adversely affected by contamination of the cuttings caused by cavings within the wellbore, by recirculated solids which are not removed by solids control equipment, and by other contaminants such as unwashed drilling mud, by cement, oil, grease, and metal shavings.
The depth of a cutting can be calculated by correlating the depth of the drill bit with the drilling mud velocity within the wellbore. This method does not differentiate between cuttings of different sizes because of the transport ratio previously described, and this method inherently incorporates certain measurement errors. Another method determines the depth of a cutting by visually correlating the mineralogy of the cutting to samples procured from an offset well. This technique requires the existence of preexisting stratigraphic information which may not be available.
Accordingly, a need exists for a method and apparatus which can efficiently measure cuttings, and which can correlate cutting samples with the original wellbore depth of such cuttings.